Satellite System Fundamental

SATELLITE COMMUNICATIONSYSTEM FUNDAMENTAL

APSAT 2006Le Meridien Hotel, Jakarta

May 30th, 2006

Training on

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• Satellite may have different meanings, however in this context the satellite means a machine that has been sent into space and goes around the Earth, moon etc., used for radio, television, and other electronic communication (Longman Advance American Dictionary)

• Basic concept of satellite transmission

Satellite Communication OverviewSatellite Communication Overview

Earth StationA

Earth StationB

Full two-way link = 1 circuitOne-way link (A to B) = 1 channelTwo-way link (A to B) = ½ circuit

½circuit

½cir

cuit

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Types of Satellite OrbitSatellite Orbit

• Transmission latency or delay of 250 millisecond to complete up and down link

• Higher cost and more difficult launch deployment to GEO

• Antennas must be of larger aperture size to provide high flux density service and narrow beams for frequency reuse

• Poor look angle elevations at higher latitudes

• User terminals do not have to track satellite• Only a few satellites can provide global

coverage• Maximum life-time (15 years or more)• Do have to worry about Van Allen Belt Radiation• Often the lowest cost system and simplest in

terms of tracking and switching

GEO

DisadvantagesAdvantagesOrbit

• Geo = Earth + synchronous = moving at the same rate• 22,300 miles located directly above the equator• Similar rotation period with earth on its axis• So, a satellite in GEO always stays directly over the same

spot on Earth• can also be called a GeoSTATIONARY Orbit

GeosynchronousEquatorial Orbit

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Types of Satellite Orbit

• More satellites to deploy than GEO• More expensive launch costs than GEO

because of more satellites• Ground antennas are generally more

expensive and complex• Increased exposure to Van Allen Belt

radiation• Coverage of low traffic areas (i.e., oceans

deserts, jungles)

• Less latency and delay than GEO (but greater than LEO)

• Improved look angle to ground receivers• Improved opportunity for frequency re-use as

compared to GEO (but less than LEO)• Fewer satellites to deploy and operate and

cheaper TTC&M systems than LEO (but more expensive than with GEO)

• Longer in-orbit lifetime than LEO systems (but less than GEO)

MEO

• Larger number of satellites (50 to 70)• Not effective solution for national or

regional coverage• Extensive coverage of min. traffic areas• Higher launch costs to deploy• Harder to deploy and operate - higher

TTC&M costs• Much shorter in-orbit lifetime due to

orbital degradation

• Low latency or transmission delay• Higher look angle (especially in high-latitude

regions)• Less path loss or beam spreading• Easier to achieve high levels of frequency re-

use• Easier to operate to low-power/low-gain ground

antennas

LEO

DisadvantagesAdvantagesOrbit

Satellite Orbit

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Types of Satellite Orbit

• GEO is at 22,230 miles or 35,836 kilometers or a tenth of the way to the moon (1/50 g). Exactly one orbit of earth every 23hr 56 min. It has 250 ms delay latency

• MEO can be anywhere from 8000 to 12000 kilometers. Above van Allen belts. Some Doppler shift concerns. 60 to 80 ms latency.

• LEO can be anywhere from 700 to 1500 kilometers. Just below van Allen Belts. Minimal latency

Satellite Orbit

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Types of Satellite Communication

Satellite Communication Overview

• Fixed Service Satellite (FSS) – is any satellite service where the ground station does not change

location frequently

– used chiefly for broadcast feeds for television and radio stations and networks, as well as for telephony, data communications, and also for Direct-To-Home (DTH) cable and satellite TV channels

– FSS satellites operate at a lower power than DBS satellites, requiring a much larger dish than a DBS system, usually 3 to 8 feet for Ku band, and 12 feet on up for C band (compared to 18 to 24 inches for DBS dishes). Also, unlike DBS satellites which typically use circular polarization on their transponders, FSS satellite transponders use linear polarization

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Types of Satellite CommunicationSatellite Communication Overview

• Mobile Service Satellite (MSS) – is a satellite system which use portable

terrestrial terminals

– The MSS terminals may be mounted on a ship, an airplane, an automobile, or even be carried by an individual. It will enable phone service anywhere on the globe

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• Broadcast Service Satellite (BSS)– A term used to refer to satellite television broadcasts intended for home

reception, also referred to as direct-to-home signals.

– It covers both analogue and digital television and radio reception, and is often extended to other services provided by modern digital television systems, including video-on-demand and interactive features. A "DBS service" usually refers to either a commercial service, or a group of free channels available from one orbital position targeting one country

– There is a terminology confusion, however ITU original frequency allocation plan for Europe and the Soviet Union from 1977 introduced a concept of extremely high power spot-beam broadcasting which they termed DBS, although only a handful of the participating countries even went as far as to launch satellites under this plan, even fewer operated anything resembling a DBS service

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Example : Mobile SatelliteExample : Fixed Satellite

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Spectrum AllocationSatellite Communication Overview

FIXED Satellite Services FSS

• C-Band (6/4 GHz)

• Ku-Band (14/12 GHz)

• Ka-Band (30/20 Ghz)

• Q/V Bands (48/38 GHz)

• W-Band (60 GHz)

MOBILE Satellite Services MSS

• 1.6/1.5 MHz

• 2.1/2.0 MHz

• 2.6/2.5 MHz

• 30/20 MHz

Feeder Links in the FSS bands

BROADCAST Satellite Services BSS

• 18/12 MHz

• 2.6/2.5 MHz

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Planned and Unplanned BandSatellite Communication Overview

Based on ITU-RR, there are two groups of frequency bands:• Planned Band is a frequency band governed by ITU and

assigned to any countries so that all countries will have on-orbit slot with specified frequency and coverage in the country territorial, and no requirement for coordination. This is primarily given for developing countries.

• Unplanned Band is a frequency band provided by ITU based on request and need from a country and treated by first come first serve philosophy. This requires coordination between countries and between operators.

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Planned and Unplanned BandSatellite Communication Overview

Location used : 123E (L and EC) by ACeS.Location not yet used: 80.5E and 135E (L & EC) by ACeS

NoneMSS

Location used: 108E (C and EC), 118E (C) by TELKOM, 113E, 150.5E (C, EC, Ku) by Indosat & PSN, 146E (C, EC, Ku) by PSN.Location not yet used: 144E (C and EC) by PSN, 118E (EC) by TELKOM, 118E (Ku) by INDOSAT

(App. 30B).Slot : 115,4EFrequency band : 4 500 - 4 800 MHz (downlink) & 6725 - 7 025 MHz (uplink); 10.70 - 10.95 GHz and 11.20 - 11.45 GHz (downlink) & 12.75 - 13.25 GHz (uplink).Disadvantages : earth station and satellite are limited, non-standard frequency range, frequency downlink 4500 – 4800 MHz (14 transponder) interfered with terrestrial frequency, and lower availability for Ku-band.

FSS

(Article-5 ITU)Location used: 107. E (S-band) by Indostar.

(App. 30 and 30 A)Slot : 80.2E, 104E , 108E , 110.4E,113E , 137E dan139.5E.Frequency band : For Region-3 (Asia including Indonesia), 11.7 – 12.2 GHz and feeder link is 17.3 – 18.1 GHz.Disadvantage : the spot beam is pointed to various location and difficult to implement to cover Indonesia.

BSS

Unplanned BandPlanned BandService

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Satellite ComponentsComponents of Satellite Communication

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Mission ConsiderationSat Com Engineering

• Purpose of the satellite– Communication, Broadcast,

Mobile, etc– Number of transponders or

channels

• Coverage Target– Global / Hemisphere– Regional– Domestic– Spot area

• Frequency operation– S, X, L, C, Ku, Ka, etc– Match with ITU frequency

allocation

• Satellite Reflector– Shaped or Phased Array– Single or multi reflector– Side lobe issue

• Orbital slot– GEO vs. Non-GEO– Elevation angle from earth station

• Design and mission life time– Redundancy scheme and

Reliability target in end of life– Suit with business plan

• Launch Vehicle– Single or Dual launch– Inclination injection

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Major Subsystem of Satellite

• Bus subsystem

• Communication or Payload subsystem

Sat Com Engineering

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Bus Subsystem

• The bus is the part of the satellite that carries the payload and all its equipment into space and contains equipment that allows the satellite to communicate with Earth.

• The bus holds all the satellite's parts together and provides :– Telemetry and Monitoring system is used to know the health status of the

satellite. All status of module and sensors are collected using telemetry database system and transmitted to earth station control

– Major subsystem are TCR (telemetry, command, and ranging), electrical power, mechanical (thermal and structure), attitude control, propulsion, and command data handling

– Highly automated system with performance alarms in case of telemetry data out of spesification

Sat Com Engineering

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Bus Subsystem• Satellite has the electric power source from Solar Arrays and Batteries

and a power converter to provide a DC voltages to all modules• Solar array

– Comprise of several panels– Need to have the capability to track to the sun through Solar Array Drive

Mechanism (SADM) in order to get optimum energy – Solar panels on body stabilized satellites more efficient that on spinners– Comprised up to hundreds of solar cells, which has the material from

Gallium Arsenide (Ga As) and Silicon (Si)

• Battery– Long life batteries needed to support mission life– Required to be able to supply up to 1 hour during eclipse season– Must be managed and discharged to perform over long lifetime– Material of its cells are Nickel Cadmium (Ni Cad), Nickel Hydrogen (NiH2),

and Lithium Ion (Li)

Sat Com Engineering

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Bus Subsystem

• The function of TCR is provided by TCR antenna, basebandmodules, on-board computer equipped with flight software, and Database Handling.

• During operation, TCR antenna can use omni, beacon, or dish antenna. It is depend on the design requirement of the system

• All process of commanding, receiving telemetry, and tracking arecontrolled and monitored from a dedicated Ground Control Systems. A backup is quite necessary to anticipate a failure including force major incidents

• Attitude control is required to keep the satellite inside the ‘box’requirement

Sat Com Engineering

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Bus Subsystem

• The status of satellite attitude may be provided by Star, sun and earth sensors or RF orientation system

• The attitude control can be provided by – Thrusters or arc jet – to correct the inclination (North and south) and

the drift (East West)

– Momentum wheel – to maintain accurate pointing

Sat Com Engineering

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Communication or Payload Components

• The payload is all the equipment a satellite needs to do its job.• The payload is different for every satellite depend on its mission. • Also known as transponder which has a mechanism to receives RF

signal, filters, amplifies (TWTA or SSPA) and converters it to downlink frequency.

• Basically, two types of satellite communication :– Analog Bent-Pipe– Digital on-board processing

• On-board switching interconnects spot beams• On-board processing and signal regeneration recreates signal without

distortions (signal enhancement)• In the future - On-Board signaling can provide “intelligent

telecommunications services” (I.e. ATM switch) to support IP over ATM services.

Sat Com Engineering

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Payload Block Diagram (Simple)

InputCircuit Receiver

IMUX (InputMultiplexer)

AntennaReceive

To OutputSection

High Power Amplifier

OMUX (OutputMultiplexer)

AntennaTransmit

From InputSection

• Basically, the payload comprises of two main sections, that is the input section and the output section

• The requirement of the mission drive the complexity of payload component configuration

Sat Com Engineering

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Satellite Communication NetworkTo a more complex onePoint to Multi-pointMulti-point to Multi-point

Sat Com Engineering

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Satellite Communication TermsSat Com Terms

• The field of satellite communications is based on a large number of basic terms and concepts, and requires a knowledge of mathematic and physical theorems that most are straightforward

• Some terms to understand are – Units : dB, dBm, Hz

– Ratios : G/T, C/N, Eb/No

– Constant : Boltzman, speed of light

– Terms : antenna gain, wavelength, BER, flux density, free-space loss, link analysis/budget, back-off, linearization

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• Decibel (dB)– A decibel is a logarithmic scale measure that is used in

communications and particularly useful for satellite communications because it allows for a significant range of power variations. Due to the long path of Geo Satellites the signal level is very low and, consequently the antenna gain and power need very high numerical number. Both are easier expressed in decibel

– Mathematical term• From numeric to decibel numbers : a (dB) = 10·log10b , or

• From decibel to numerical numbers : b = 10(a/10

• Where a is a decibel number and b is a numerical number


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(Cont’d dB)

– Examples : an amplifier has 3 dB gain means it can amplify the input power as many as two times

• dBm and dBW– A dB measurement in mili Watt or Watt

– 0 dBW = 1 Watt and 30 dBm = 1000 mW = 1 W, thus 0 dBW = 30 dBm

– Examples : 5 Watt = 7 dBW and 5000 mW = 30 + 7 = 37 dBm

• Hertz (Hz)• Hertz is a unit of frequency (f) and can be expressed as the number of

cycles per second

• Speed of light (c) equals to 2.997925 x 108 m/s

• f = c / λ, where λ is a wavelength in meter


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• Gain of (a Parabolic) Antenna– A key performance parameter of an antenna

– The larger the aperture of an antenna the more concentrated the beam and the higher the gain

– Also as you go to higher frequencies, the antenna can send or catch more of the transmitted or received signals

– Mathematical terms : G = η·(4πA)/(λ2), or if the aperture is circular can be expressed as G = η·(πD/λ)2

– Where : G = Gain of parabolic antenna ; η = antenna aperture efficiency ; A = antenna aperture area (m2) ; λ = radiation wavelength (m) ; c = speed of light (m/s) = 2.997925 x 108 m/s ; f = radiation frequency in Hz


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• Effective Isotropic Radiated Power (EIRP)– Isotropic refers to a signal sent in all directions equally

– It is simply the power generated by the high power amplifier (HPA) multiplied by the antenna gain and taking into account the transmission line that connect the output of HPA to the antenna feed

– Mathematical term : EIRP = Pt·Gt/Lf or in dB terms : EIRP = Pt + Gt - Lf

• G/T– It is simply as the ratio of receive antenna gain divided by the noise

temperature of the receiving system

• Boltzman Constant (k)– It is used extensively in calculating a noise power bandwidth

– Equals to 1.38 x 10-23 J/K or in dB terms is -228.6 dBJ/K


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• Carrier Signal to Noise Ratio (C/N)– Carrier Signal to Noise is a measure of the transmitted power of a

carrier in relation to the noise or interference in the carrier band

– Unit is in dB

– In satellite communication, C/Nuplink is usually higher than C/Ndownlink , so the C/N total is close to C/N downlink or said to be downlink limitted.

• Eb/N0

– Eb/N0 is the ratio of the power per data bit to the noise power density per Hz. This is the basis for determining the quality of a digital channel

– N0/2 is a white noise power spectral density which is uniform across the frequency

– N0 equals to Boltzman constant multiplies to noise temperature of the noise source measured in Kelvin (1º Kelvin = 273 ºC), or N0 = k·Ts


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• Bit Error Rate (BER)– Bit Error Rate is the determination of Quality of Service in a digital

system

– An error over a million bit means the BER has 10-6

• Free Space Loss (L)– While the signal propagates toward the space, it will experience an

attenuation and a change in amplitude, phase, or frequency due to traveling distance and atmospheric condition

– Mathematical terms : L = (4πd/λ)2, where d is a slant range


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• Power Flux Density (PFD)– The flux density is the power flow per unit surface area. The greater the

distance travel the flux density decreases by the square of the distance traveled

– The power flux density is thus a vector quantity determined by how little of a sphere’s surface it illuminates. The tighter the antenna beam the higher the received power flux density

– Mathematical term : PFD = EIRP/(4πd2L), expressed in dBW/m2

• Basic Link Budget• C/N total = [ (C/N uplink)-1 + (C/N downlink)-1 ]-1

• C/N uplink = EIRPes – La - Lu + G/Tsat – k – B

• C/N downlink = EIRPsat – La – Ld + G/Tes – k – B

Satellite Communication Terms

where La is antenna tracking loss and atmospheric attenuation, sat is from satellite, and es is from earth station

Sat Com Terms

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• Multi carrier Operation– Multi carrier operation in a single amplifier creates an inter-modulation

interference

– The input level of an amplifier is required to be reduced (back-off) to certain level, typically until linear region, until the inter-modulation interference is not a significant contribution to the overall link system

– In the non-linear region, the amount of Input Back-Off (IBO) is more than the amount of Output Back-Off (OBO). It will depend on each characteristics of each amplifier

– SSPA is typically more linear than TWTA. Nowadays, almost all TWTAsare equipped with a linearizer in order to extend the linear region

Multi Carrier OperationSat Com Terms

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Multi Carrier Operation

-30 -20 -10 0

Power Input Relative to Saturation (dB)

0

-10

-20

-30

-40

Pow

er O

utpu

t Rel

a tiv

e to

Sat

urat

ion

(dB)

Input Backoff

12 dBOnput Backoff

Saturation

1 dB Compression Point

Linear Region

Sat Com Terms

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Multi Carrier Operation

(2f1-f2) f1 f2 (2f2-f1)

3rd order inter-modulation

f

(3f1-2f2) (3f2-2f1)

5th order inter-modulation

Inter-modulation product from f1 and f2 carriers

Sat Com Terms

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Multiple Access Technique

FDMA - transmit in frequency slots :• most often used with analog systems• allows many users to share a transmission bandwidth by assigning each of

the users a specific part of the transponder’s bandwidth such that the sum of all user bandwidths is allocated except for needed guard-bands between the carriers

• Demand Assigned Multiple Access (DAMA) and Pre-Assigned Multiple Access (PAMA)

FDMA, TDMA, and CDMA are the most common way by which multiple user share the use of a satellite or even share a transponder

Satellite Communication System Engineering

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Frequency

Tim

e

Different channels use some of the bandwidth all of the time.

1 2 3 4 5

• Allows smaller receiver bandwidth (less noise power)• Smaller maximum transmit power requirements• Operates both in star and mesh topologies

Frequency Division Multiple AccessSatellite Communication System Engineering

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• PAMA (Pre Assigned Multiple Access) - implies that the VSATs are pre-allocated a designated frequency. Equivalent of the terrestrial leased line solutions, PAMA solutions use the satellite resources constantly. Consequently there is no call setup delay which makes them most suited for interactive data applications or high traffic volumes . As such PAMA is used typically to connect high data traffic sites within an organization. SCPC (Single Channel Per Carrier) refers to the usage of a single satellite carrier for carrying a single channel of user traffic. The frequency is allocated on a pre-assigned basis in case of SCPC VSAT's. The term SCPC VSAT is often used interchangeably with PAMA VSAT.

• DAMA (Demand Assigned Multiple Access) - network uses a pool of satellite channels, which are available for use by any station in that network. On demand a pair of available channels are assigned so that a call can be established. Once the call is completed, the channels are returned to the pool for an assignment to another call. Since the satellite resource is used only in proportion to the active circuits and their holding times, this is ideally suited for voice traffic and data traffic in batch mode. DAMA offers point to point voice, fax, and data requirements and supports video conferencing.

FDMA – Implementation OptionsSatellite Communication System Engineering

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TDMA - transmit in time slots :• most common form of digital systems • users are separate by being assigned a particular time slot rather than a

particular frequency. Larger users can be assigned longer time slots (even though these time slots are measured in milliseconds). This approach is useful for satellites since it allows for processing time for on-board processing and interconnection of the uplink and down link beamsappropriate for each user

• Fixed Assigned TDMA, ALOHA & Slotted ALOHA, and Dynamic Reservation


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Time Division Multiple Access

FrequencyTi

me

Different channels use all of the bandwidth some of the time.

Predictable time assignments.

1

2

3

1


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CDMA - in a combination of time slots and code :• the newest form of multiplexing and this is used for satellites and for mobile

terrestrial wireless systems as well.• CDMA uses a combination of time slots and code to separate out multiple

users of a satellite transponder. With very efficient coding techniques a large number of users can share the available spectrum with coding being used to pick out the signal meant for each user.

• More immune on interference and multipath effect compared to other MA techniques (FDMA or TDMA)

• Implementation Choices are Direct Sequence and Frequency Hopping


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Code Division Multiple Access

Frequency

ti me

Different channels use all of the bandwidth all of the time.

Channels use different codes.Other channels cause

noise-like interference.


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Direct Sequence Spread Spectrum

+1

-1time

time

time+1

-1-1-1

+1 +1 +1

Traffic(9 Kbps)

SpreadingSignal27 Kcps

TransmittedSignal27 Kcps

+1 +1+1

-1 -1

Modulator

Transmitter

Channel

Spreading (PN) Code

Tc << T

Carrier Recovery Demod

Receiver

Spreading (PN) Code

Data (T)

Data (T)

Synch

Interference

Original Data Signal

Narrowband Filter

Other SS Users

Demodulator Filtering

ISI

Modulated Data

Data Signal with Spreading

Narrowband Interference

Other SS Users

Receiver Input

ISI

Modulator

Transmitter

Channel

Spreading (PN) Code

Tc << T

Carrier Recovery Demod

Receiver

Spreading (PN) Code

Data (T)

Data (T)

Synch

Interference

Original Data Signal

Narrowband Filter

Other SS Users

Demodulator Filtering

ISI

Modulated Data

Data Signal with Spreading

Narrowband Interference

Other SS Users

Receiver Input

ISI

time

time+1

-1-1-1

+1 +1 +1DespreadingSignal27 Kcps

ReceivedSignal27 Kcps

+1 +1+1

-1 -1

+1

-1

timeRecoveredTraffic9 Kbps

• If the proper source is transmitting...

• ...and the receiver has the correct despread sequence...

• ...and the sequence is properly synchronized...

• ...the original message is recovered.


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More Technique for Frequency Re-Use

SDMA – using different space• The simplest way to have multiple access by just keeping beams

apart and then reuse frequencies by spatial separation.• Implemented by using a Multi-beam antenna which creating

separate beams that do not overlap– Very large scale antennas with a complex feed system can create

many beams for frequency re-use. The Iridium system, for instance, created 48 different beams

– A phased array antenna uses a number of different elements that are controlled by a processor to “electronically shapes” the beam. Thus it can “electronically track” a satellite but a parabolic antenna must physically track or constantly point directly at the satellite.


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Space Division Multiple AccesSatellite Communication System Engineering

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More Technique for Frequency Re-Use

Circular and Orthogonal Polarization• The two ways to polarize signals to allow the re-use of frequencies in

adjacent or overlapping beams are circular and orthogonal polarization (much like polarized sunglasses).

• Orthogonal or cross pol: Signals at 90° angles• Circular pol: Signals in right hand circular and left hand circular directions

(in 26 to 32 dB discrimination)


Linear / Orthogonal Circular

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Modulation

• Modulation is a technique used to transmit baseband information (modulated signal) over a bandpass channel (modulated signal) in a manner suitable for transmission

• The frequency of the bandpass is much higher that the frequency of the baseband signal


V(f)V(f)

0 Bf f

fcfc - B/2 fc + B/2

Ideal

Real

Baseband Bandpass

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Modulation• Modulation can be done by varying either the Amplitude,

Frequency, and Phase, or the combination of those• Demodulation is the reversing process to extract the original

message• In satellite communication, the far distance and hostile environment

require a robust modulation technique to achieve the best possible quality and consider an efficient use of resource (power and frequency)

• Two types of modulation– Analog Modulation : the input is a continuous signal and has 3

fundamental types, i.e. Amplitude Modulation (AM), Frequency Modulation (FM), Phase Modulation (PM)

– Digital Modulation : the input is time sequence of symbols or pulses and has 3 fundamental types, i.e. Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), Phase Shift Keying (PSK)


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Amplitude ModulationSatellite Communication System Engineering

Modulating and Carrier Signal

)2cos()](1[)()(

)2cos(

tftmAtstm

tfA

ccAM

cc

π

π

+=

:Signal AMThe

:Signal Message Modulating

:Signal Carrier

Baseband Bandpass

TransmissionMedium

Original Signal

Transmitter Site Receiver Site

Demodulator

fmfmfc

Modulator

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Amplitude ModulationSatellite Communication System Engineering

• Frequency : fc >> fm

+⋅=

21

2a

ctnPP• Power relationship :

The ideal would be that most of the signal power is used to transmit information (na >> 1), however, na must remain < 1 to avoid loss of information.

Conditions and Relationship

na is known as the mod index, the ratio of amplitude of the input signal to the carrier

Notes :fm is the frequency of baseband signal

Pc is the transmitted power in the carrier

fc is the frequency of carrier

Pt is the transmitted power in s(t)

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FM and PM ModulationSatellite Communication System Engineering

In practice, a good approximation to the bandwidth in angle modulation is known as the Carson’s rule

( )

⋅⋅

⋅=

∆⋅

=

⋅+⋅=

FMforB2B

PMforB12

πβ

β

mf

mp

T

AnfAn

B

Both FM and PM require greater bandwidth than AM

Np is the phase modulation indexNf is the frequency modulation index

Where :

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Comparison of FM and AMSatellite Communication System Engineering

• FM is better dealing with noisy and multipath environment– not very sensitive in amplitude variation

– atmospheric or impulse noise cause rapid fluctuations in the amplitude of the received signal

– small-scale fading cause amplitude fluctuations as we have seen earlier

• FM is able to trade between SNR ratio to occupied bandwidth• The relationship between received power and quality is non-linear

– Rapid increase in quality for an increase in received power

– Resistant to co-channel interference (capture effect)

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Digital ModulationSatellite Communication System Engineering

• The information is consisting of discrete signal and assumed to be binary, that is, 0 and 1, or in more complex ones is in M-ary form

• Digital modulation has extensive advantages compared to analog modulation :

– Less usage in bandwidth and power

– More immune to channel impairments

– Easier multiplexing of various sources of information: voice, data, video.

– Be able to reduce error by error-control codes

– Enables encryption of the transferred signals to provide more secure link

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Digital ModulationSatellite Communication System Engineering

• The modulating signal is represented as a time-sequence of symbols or pulses

• Every symbol can have many states and the relationship can be drawn as follows :

M = 2 k or k = log2 M

• For example :

00 ; 01 ;10 ; 1124

000 ; 001 ; 010 ; 011 ; 100 ; 101 ; 110 ; 11138

0 ; 112

M Statesk(bits / symbol)

M

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Digital Modulation and DemodulationSatellite Communication System Engineering

Mathematical representation of any modulated signal

s(t) = + A(t) cos [ωct + φ(t)]

s(t)A(t) cos φ(t) cos ωct A(t) sin φ(t) sin ωct

in-phase quadrature

Or if it can be expanded by using simple mathematic rule

-=

represents binary digit “1”

s(t) = - A(t) cos [ωct + φ(t)] represents binary digit “0”

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Digital DemodulationSatellite Communication System Engineering

• Optimum demodulator can detect one out of M possible transmittedsignals in an M-ary signaling with minimum probability of error

• The coherent demodulation is synchronized with the transmitter and can only occurred when the initial phase of the transmitted signal is known perfectly at the receiver

• The implementation of coherent demodulation– difficult to obtain in a rapidly fading environment

– increases receiver complexity

• Differential detection uses the previous symbol for the reference– eliminates need for coherent reference

– entails loss in power efficiency (up to 3 dB)

– Doppler causes error floor, however typically small for high bit rates

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Efficiency of ModulationSatellite Communication System Engineering

• Efficiency in any transmission involves two main parameters thatare power and bandwidth

• Power efficiency is the ability of the modulation technique to preserve fidelity of the message at low power levels

Power Efficiency : ηP = f (Eb/No for certain PER)

where : Eb/No = ratio of signal energy per-bit to noise density PER = probability of error

• Bandwidth efficiency is a parameter to show how efficient the allocated bandwidth is utilized

Bandwidth Efficiency : ηB = R/B

R = data rate in bps B = occupied RF bandwidth in Hz

• Trade-off between power and bandwidth efficiency

– Add error control code ηP increase ; ηB decrease– Increase M-ary modulation ηP decrease ; ηB increase

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Data Rate and BandwidthSatellite Communication System Engineering

• In a noiseless (ideal) environment, Nyquist theorem provides the the relationship between the channel bandwidth and maximum data rate

mBC 2log2=

• Shanon theorem provides the relationship between channel bandwidth and maximum data rate that can be transmitted in Gaussian noise present

)1(log 2max NS

BC

B +==η

where : C = channel capacity or maximum data rate (bps)M = number of finite states in a symbol of transmitted signal

S/N: signal-to-noise ratio

Probability

p(y0) p(y1)

P(e|0)P(e|1)

-V 0 V y, receiver input

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ASK and OOKSatellite Communication System Engineering

• Amplitude Shift Keying is a linear (non-constant envelope) modulation technique

• The difference between bit “0” and “1” are indicated respectively by low and high amplitude.

• On Off Keying is the same form as ASK, except the bit “0” is represented only by no voltage

Figure : example of ASK Figure : example of OOK

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Binary Phase Shift KeyingSatellite Communication System Engineering

• The phase of the carrier changes between two values (state “0” and state “1”) separated by π radians or 180º

• The carrier can be turned on and off by a carrier on/off signal to determine when the modulator will emit the PSK signal

• Simple to implement, however inefficient use of bandwidth

• Very robust and used extensively in satellite communication

S1(t) = A cos(wct + θ) represents “1”

represents “0”S1(t) = A cos(wct + π + θ)

= - A cos(wct + θ)

Q

0 1

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BPSK and DPSKSatellite Communication System Engineering

• Differential Phase Shift KeyingThe presence or absence of a transition can be used to encode data, in which a 1 is represented by the presence of a transition (180º phase shift), and a 0 is represented by the absence of a transition (0º or no phase shift)

1 1 0 01 1• BPSK example :

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Phase Shift KeyingSatellite Communication System Engineering

• PSK is also a linear modulation technique

• Easier to understand the concept by using constellation diagram

– Bandwidth occupied by the modulation increases as the dimension of the modulated signal increases

– Bandwidth Bandwidth occupied by the modulation increases as the dimension of the modulated occupied by the modulation decreases as the signal points per dimension increases (getting more dense)

– Probability of bit error is proportional to the distance between the closest points in the constellation, that is Bit error decreases as the distance increases (sparse)

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Quaternary Phase Shift KeyingSatellite Communication System Engineering

• 1 symbol is represented by 2 bits, or said 2 bits per symbol

• Two times bandwidth efficiency with the penalty of more complex receiver

“10”π/4

“00”3π/4

“01”5π/4

“11”7π/4

sin(wct + θ)

cos(wct + θ)

S1(t) = s10(t) = A cos(wct + π/4 + θ)

S2(t) = s00(t) = A cos(wct + 3π/4 + θ)

S3(t) = s01(t) = A cos(wct + 5π/4 + θ)

S4(t) = s11(t) = A cos(wct + 7π/4 + θ)

Corresponding transmitted signal

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M-ary PSK ModulatorSatellite Communication System Engineering

Serial-to-

parallelconverter

I & Qsignal

generator

OSC

90º

Σ

I signal

Q signal

cos ωct

sin ωct

2

m

1Binarydata M-ary PSK

63

M-ary PSK DemodulatorSatellite Communication System Engineering

Carrierrecoverycos ωct

90°

r(t)

Low-passfilter

Low-passfilter

Clockgeneration

Digitallogic

12

m

Parallel-to-

serialconverter

Binarydata

I signal

Q signal

64

Spectrum of M-ary PSKSatellite Communication System Engineering

0

-20

-40

-60

Pow

er sp

ectra

l den

sity

(dB

)

1/2Tb 1/Tb 3/2Tb 2/Tb

f - fc

2-PSK4-PSK

8-PSK

fc+ fc+ fc+ fc+fc

65

Frequency Shift KeyingSatellite Communication System Engineering

• FSK has a constant envelope regardless of the variation in modulating signal which has characteristics as follows :– Better deal with random noise and fluctuations due to fading

– More power efficiency, but less bandwidth efficiency

• The frequency of the carrier in Bi-FSK is changed according to the message state, for example high is state “1” or low “0” :

0)(bit Tt0 1)(bit Tt0

b

b

=≤≤∆−==≤≤∆+=

tffAtstffAts

c

c

)22cos()()22cos()(

2

1

ππππ

66

Quadrature Amplitude ModulationSatellite Communication System Engineering

• QAM transmits two signals simultaneously on the same frequency with one shifted 90º with respect to the other

• QAM is a combination of amplitude and phase modulation

( ) ( )tfttftts cc ⋅⋅⋅⋅+⋅⋅⋅⋅= ππ 2sin)(d2cos)(d)( 21

16-QAM2 amplitudes, 8 phases

67

Forward Error CorrectionSatellite Communication System Engineering

Data block

k symbols

Data Redundant

FEC encoding

k n-k

n

Data block

kFEC decoding

Each symbol contains a predefined number of bits

Noisy channel

Code rate= k/n

68


• Many protocols operate by only detecting errors and ask for retransmitting• The problem with retransmission in GEO satellite communication is the

round trip delay that may be too long for high speed and real-time application

• To overcome this problem, it is better to transmit enough additional data to allow receiver to correct some errors

• Example: data with m × n bits:

data checksum(n–1)(m–1) bits m + n – 1 bits

dn,mdn,m–1…dn, 1Col. parity

dn–1,mdn–1,m–1…dn–1, 1

………

d2,md2,m–1…d2, 1

d1,md1,m–1…d1,1

Row parity

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• For even parity, each row and column should have an even number of 1’s• The row and the column together indicate where a single error occurred• This scheme can correct a single error, and detect other combinations of errors• Much more powerful error correcting codes exist

okokokokokerror

ok110101

ok101110

ok001111

error010101

okokokokokerror

ok110101

ok101110

ok001111

error0101010 0 1 0 1

1 1 1 1 0

0 1 1 1 0

Data transmittedm=4 and n=3

Add Redundant Datain rows and columns

1 0 1 0 1

0

0

1

1

Noisy channel

70

THANK YOU

Documents

Satellite System Fundamental